Chlorine dioxide, ClO2, is a yellow to reddish-yellow manufactured reactive gas that has a broad number of industrial applications including, for example, bleaching wood and disinfecting industrial and municipal waters. Due to its reactive nature, chlorine dioxide is generally made in situ by chemical or electrochemical reaction. Chlorine dioxide does not occur naturally in the environment and is generally not suitable for commercial storage or transport as a gas because it is explosive under pressure. One of the most important physical properties of chlorine dioxide is its high solubility in water, particularly in chilled water. Chlorine dioxide dissolved in water does not hydrolyze to any appreciable extent but remains in solution as a dissolved gas.
Chlorine dioxide can be generated from a variety of compounds including the oxychlorine anions such as chlorite or chlorate. For most commercial applications, chlorine dioxide can be formed by sodium chlorite reacting with gaseous chlorine (Cl2(g)), hypochlorous acid (HOCl), or hydrochloric acid (HCl). UV irradiation of chlorite solutions will also generates chlorine dioxide; however it is also well known that chlorine dioxide solutions are commonly destroyed by UV light in water basins that are exposed to sunlight or bright fluorescent lights.
Chlorine dioxide is used in a diverse range of applications including as a bleaching agent at paper manufacturing plants, and in public water treatment facilities to make water safe to drink. A common application of chlorine dioxide in drinking water in the United States has been for control of tastes and odors associated with algae and decaying vegetation. Chlorine dioxide is also effective in destroying taste and odor producing phenolic compounds. Chlorine dioxide can be used to oxidize both iron and manganese. Chlorine dioxide reacts with the soluble forms of iron and manganese to form precipitates that can be removed through sedimentation and filtration. Chlorine dioxide is more effective as a disinfectant than chlorine against water borne pathogenic microbes such as viruses, bacteria and protozoa in most circumstances. In 2001, chlorine dioxide and chlorite were used to decontaminate a number of public buildings following the release of anthrax spores in the United States.